linux-kernel - [PATCH 3/7] openrisc: restore call-saved regs on sigreturn

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Date:   Fri, 16 Sep 2016 23:43:01 +0900
From:   Stafford Horne <shorne@...il.com>
To:     Jonas Bonn <jonas@...thpole.se>,
        Stefan Kristiansson <stefan.kristiansson@...nalahti.fi>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     linux-kernel@...r.kernel.org, Stafford Horne <shorne@...il.com>
Subject: [PATCH 3/7] openrisc: restore call-saved regs on sigreturn

From: Jonas Bonn <jonas@...thpole.se>

Return to userspace via _resume_userspace instead of via syscall return
path for the rt_sigreturn syscall.

I'll rework this comment more later, but this patch needs testing.

Old comment from previous patch:

The sigreturn syscall is more like a context switch than a function call;
it entails a return from one context (the signal handler) to another
(the process in question).  For a context switch like this there are
effectively no call-saved regs that remain constant across the transition.

This patch restores the call-saved regs from pt_regs before returning from
the syscall, effectively restoring the context that the process had before
being interrupted by the signal handler.  Restoring the call-saved regs
in this way allows us to return to userspace via the usual syscall fast
path.

Reported-by: Sebastian Macke <sebastian@...ke.de>
Signed-off-by: Jonas Bonn <jonas@...thpole.se>
Signed-off-by: Stafford Horne <shorne@...il.com>
---
 arch/openrisc/kernel/entry.S | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/arch/openrisc/kernel/entry.S b/arch/openrisc/kernel/entry.S
index fec8bf9..572d223 100644
--- a/arch/openrisc/kernel/entry.S
+++ b/arch/openrisc/kernel/entry.S
@@ -1101,8 +1101,16 @@ ENTRY(__sys_fork)
 	 l.addi	r3,r1,0

 ENTRY(sys_rt_sigreturn)
-	l.j	_sys_rt_sigreturn
+	l.jal	_sys_rt_sigreturn
 	 l.addi	r3,r1,0
+	l.sfne	r30,r0
+	l.bnf	_no_syscall_trace
+	 l.nop
+	l.jal	do_syscall_trace_leave
+	 l.addi	r3,r1,0
+_no_syscall_trace:
+	l.j	_resume_userspace
+	 l.nop

 /* This is a catch-all syscall for atomic instructions for the OpenRISC 1000.
  * The functions takes a variable number of parameters depending on which
-- 
2.7.4